I bought a Boss OS-2 (overdrive/distortion) to use with my strat, but i found it gets some good sounds for bass too. Unfortunately you cant get any real deep tones out of it since it was designed for guitar, but it gets some great sounds, really funky overdrive (when I want to give the tubes a rest

) and some good grindy distortion.
You can always use pedals designed for guitar with bass, but some dont affect the low lows as much as you would want for a bass effect.

I really need to try myself out an English Muff'n...